1. Field of the Invention
The invention relates to a valve drive for charge exchange valves, preferably inlet valves of reciprocating internal combustion engines. The valve drive includes two cams having different stroke characteristics for slight and maximum opening, whereby cam halves of the second cam abut the first cam on both sides. A cup tappet is guided in the cylinder head which selectively engages the cam halves of the second cam. A central tappet, guided in the cup tappet, engages the first cam. A mechanical or hydraulic lash adjuster is arranged within the central tappet where it is supported against the valve. A hydraulically-switchable driving arrangement is arranged between the central tappet and the cup tappet and has radially-guided, displaceable drivers in one of the tappets. The drivers are engageable in counter contours of the other tappet within the range of the base circles of the two cams.
2. The Prior Art
Valve drives of the above-specified type with an adjustable stroke and two cams with different stroke characteristics are known from published specification WO 91/12413, especially FIGS. 4 to 6 thereof. A mechanical driving device is arranged between the cup tappet and the central tappet, for coupling the two tappets together. The driving device has driving pins, which are supported in the cup tappet, and are acted upon hydraulically and displaced radially inwardly against a spring force. The pins engage an annular surface on the central tappet, which faces the camshaft.
When the driving device is inactive, the slight stroke is transmitted to the valve by the first cam. When the device is active, the greater stroke is transmitted by the second cam. The engagement of the driving pins in the annular surfaces on the central tappet is possible only when both cams pass through their base circles. For this purpose, the base circle of the first cam has to be relatively larger than the second cam in order to assure the required play between the annular surface on the central tappet and the engagement contour of the driving pins for engagement. In the active condition of the second cam, during each pass through the base circles, a change in engagement occurs between the first cam and the driving pins on the annular surface on the central tappet. It is known also from FIGS. 5 and 6 of said published specification to provide the central tappet with a hammer-shaped engagement head projecting beyond the cup tappet, with the rolling surface of said head corresponding approximately to the diameter of the cup tappet. The head is guided between the cam halves of the cam for the minor stroke.
From U.S. Pat. No. 5,090,364, particularly FIGS. 8 to 12 thereof, there is known a tappet arrangement for two valves to be actuated in parallel, with a driving device for alternating between two cams with different stroke characteristics. A mechanical driving device, by means of which the two tappets can be coupled, is arranged within an outer tappet that engages both valves. A central tappet is displaceable within the outer tappet. When the driving device is inactive, the minor stroke is transmitted to the valves by the halves of a first cam, and when the driving device is active, the maximum stroke is transmitted by a second cam arranged between the halves of the first cam.
The driving device has balls supported in a central guide of the outer tappet. The balls are displaceable radially outwardly and engageable with an inwardly slanted annular surface on the central tappet, which faces away from the camshaft. The displacement and support of the balls in the engaging position is accomplished by means of a locking slide. The locking slide is displaceable against an opposing spring force by admission of pressure, and concentrically guided in the central guide of the outer tappet.
A driving device of the above-mentioned design, with a locking element arranged in the center, cannot be used in a switchable cup tappet having the usual dimensions and a cylindrical outer shape. The cup tappet actuates a valve by engagement, for example along its axis with an automatic lash adjuster. Furthermore, valve drives of the type specified above are known also from German Patents DE 35 45 537 A1 and DE 33 47 680 A1, where a controllable hydraulic cushion between the cup tappet and the central tappet forms the driving device.